Proteasome inhibitors have shown promise in cancer therapy as a single agent. We have recently observed that the proteasome inhibitor, PS-341, can sensitize neoplastic cells to TRAIL-mediated death. This sensitization occurred through the down-regulation of c-FLIP, an important mediator in the prevention of apoptosis. Surprisingly, this sensitizing activity was also shown to be independent of NF-kappaB in some tumor lines. We now wish to extend these findings to ascertain if proteasome inhibition can be used in conjunction with the killing potential of adoptive immunotherapy and the extensive cytoreductive conditioning that precedes bone marrow transplantation (BMT). To do this, several Specific Aims are proposed: Specific Aim 1 will extend on the in vitro studies and examine the effects of PS-341 and a genetic construct for TRAIL (Fc-TRAIL) or a recently obtained agonist TRAIL receptor antibody (anti-DR5) on advanced tumor-bearing mice. The mechanism underlying the anti-tumor effects will also be dissected using TRAIL KO mice. Specific Aim 2 will assess the effects of PS-341, with or without Fc-TRAIL or anti-DR5, in a minimal residual disease model in which the tumor-bearing mice receive a bone marrow transplant (BMT). Both syngeneic and allogeneic BMT will be used and effects on immune and myeloid reconstitution as well as tumor relapse will be assessed. In allogeneic BMT models, effects on graft-versus-host disease (GVHD) will be ascertained. Finally, in Specific Aim 3, effects of PS-341 as a means to sensitize tumor cells to T and NK cell killing will be determined using both in vitro and in vivo assays. This will culminate with combining the data attained from the previous specific aims and assessing the effects of PS-341 with NK cells and anti-DR5 as an adoptive immunotherapy regimen in resting tumor-bearing mice and in tumor-bearing mice after BMT. These results will allow for the assessment of the efficacy of proteasome inhibition with PS-341 in conjunction with immunotherapy using a variety of tumor types and in preclinically-relevant models.